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The thermal state of the lithosphere and related geothermal heat flow (GHF) is a crucial parameter to understand a variety of processes related to cryospheric, geospheric, and/or biospheric interactions. Indirect estimates of GHF in polar regions from magnetic, seismological, or petrological data often show large discrepancies when compared to thermal in situ observations. Here, the lack of in situ data represents a fundamental limitation for both investigating thermal processes of the lithosphere and validating indirect heat flow estimates. During RV Polarstern expeditions PS86 and PS118, we obtained in situ thermal measurements and present the derived GHF in key regions, such as the Antarctic Peninsula and the Gakkel Ridge in the Arctic. By comparison with indirect models, our results indicate (1) elevated geothermal heat flow (75 ± 5 mW m−2 to 139 ± 26 mW m−2) to the west of the Antarctic Peninsula, which should be considered for future investigations of ice-sheet dynamics and the visco-elastic behavior of the crust. (2) The thermal signature of the Powell Basin characteristic for oceanic crust of an age between 32 and 18 Ma. Further, we propose (3) that at different heat sources at the slow-spreading Gakkel Ridge in the Aurora Vent Field region might explain the geothermal heat flow distribution. We conclude that in situ observations are urgently required to ground-truth and fine-tune existing models and that a multidisciplinary approach is of high importance for the scientific community’s understanding of this parameter

Antarctic geothermal heat flow (GHF) affects the temperature of the ice sheet, determining its ability to slide and internally deform, as well as the behaviour of the continental crust. However, GHF remains poorly constrained, with few and sparse local, borehole-derived estimates and large discrepancies in the magnitude and distribution of existing continent-scale estimates from geophysical models. We review the methods to estimate GHF, discussing the strengths and limitations of each approach; compile borehole and probe-derived estimates from measured temperature profiles; and recommend the following future directions. (1) Obtain more borehole-derived estimates from the subglacial bedrock and englacial temperature profiles. (2) Estimate GHF from inverse glaciological modelling, constrained by evidence for basal melting and englacial temperatures (e.g. using microwave emissivity). (3) Revise geophysically derived GHF estimates using a combination of Curie depth, seismic, and thermal isostasy models. (4) Integrate in these geophysical approaches a more accurate model of the structure and distribution of heat production elements within the crust and considering heterogeneities in the underlying mantle. (5) Continue international interdisciplinary communication and data access.

Antarctic geothermal heat flow (GHF) affects the ice sheet temperature, determining how it slides and internally deforms, as well as the rheological behaviour of the lithosphere. However, GHF remains poorly constrained, with few borehole-derived estimates, and there are large discrepancies in currently available glaciological and geophysical estimates. This SCAR White Paper details current methods, discusses their challenges and limitations, and recommends key future directions in GHF research. We highlight the timely need for a more multidisciplinary and internationally-coordinated approach to tackle this complex problem.

The mid-Cretaceous period was one of the warmest intervals of the past 140 million years, driven by atmospheric carbon dioxide levels of around 1,000 parts per million by volume. In the near absence of proximal geological records from south of the Antarctic Circle, it is disputed whether polar ice could exist under such environmental conditions. Here we use a sedimentary sequence recovered from the West Antarctic shelf—the southernmost Cretaceous record reported so far—and show that a temperate lowland rainforest environment existed at a palaeolatitude of about 82° S during the Turonian–Santonian age (92 to 83 million years ago). This record contains an intact 3-metre-long network of in situ fossil roots embedded in a mudstone matrix containing diverse pollen and spores. A climate model simulation shows that the reconstructed temperate climate at this high latitude requires a combination of both atmospheric carbon dioxide concentrations of 1,120–1,680 parts per million by volume and a vegetated land surface without major Antarctic glaciation, highlighting the important cooling effect exerted by ice albedo under high levels of atmospheric carbon dioxide.

The mid-Cretaceous period was one of the warmest intervals of the past 140 million years1,2,3,4,5, driven by atmospheric carbon dioxide levels of around 1,000 parts per million by volume6. In the near absence of proximal geological records from south of the Antarctic Circle, it is disputed whether polar ice could exist under such environmental conditions. Here we use a sedimentary sequence recovered from the West Antarctic shelf—the southernmost Cretaceous record reported so far—and show that a temperate lowland rainforest environment existed at a palaeolatitude of about 82° S during the Turonian–Santonian age (92 to 83 million years ago). This record contains an intact 3-metre-long network of in situ fossil roots embedded in a mudstone matrix containing diverse pollen and spores. A climate model simulation shows that the reconstructed temperate climate at this high latitude requires a combination of both atmospheric carbon dioxide concentrations of 1,120–1,680 parts per million by volume and a vegetated land surface without major Antarctic glaciation, highlighting the important cooling effect exerted by ice albedo under high levels of atmospheric carbon dioxide.

The mid-Cretaceous was one of the warmest intervals of the past 140 million years (Myr) driven by atmospheric CO2 levels around 1000 ppmv. In the near absence of proximal geological records from south of the Antarctic Circle, it remains disputed whether polar ice could exist under such environmental conditions. Here we present results from a unique sedimentary sequence recovered from the West Antarctic shelf. This by far southernmost Cretaceous record contains an intact ~3 m-long network of in-situ fossil roots. The roots are embedded in a mudstone matrix bearing diverse pollen and spores, indicative of a temperate lowland rainforest environment at a palaeolatitude of ~82°S during the Turonian–Santonian (92–83 Myr). A climate model simulation shows that the reconstructed temperate climate at this high latitude requires a combination of both atmospheric CO2 contents of 1120–1680 ppmv and a vegetated land surface without major Antarctic glaciation, highlighting the important cooling effect exerted by ice albedo in high-CO2 climate worlds.

The most rapidly changing parts of the Antarctic Ice Sheet have been observed in Amundsen Sea Sector/Bellingshausen Sector of West Antarctica. Various processes contribute to the (in)stability of the ice sheet here. For instance, inflow of modified, warmer Circumpolar Deep Water, geothermal heat from the underlying crust and the crusts flexural response to unloading of the ice mass. Our objective is the investigation of geothermal heat flow in this sector, which is poorly constrained, albeit providing a crucial boundary condition for ice sheet models and related sea level rise predictions. We discuss airborne, high-resolution magnetic anomaly data from the Amundsen Sea Sector, to provide additional insight into deeper crustal structures related to the West Antarctic Rift System in the Amundsen/Bellingshausen sector. With the depth-to-the-bottom of the magnetic source (DBMS) estimates, we reveal spatial changes at the bottom of the igneous crust and the thickness of the magnetic layer that can be further incorporated into tectonic interpretations and serves as a proxy for geothermal heat flow estimates. The DBMS results and further available datasets (e.g. crustal thickness) are synthesized in high-resolution, thermal 2D models of the crust in two representative profiles along Pine Island and Thwaites Glacier. Because crustal parameters, such as radiogenic heat production, thermal conductivity, crustal thickness and Moho temperatures yield large uncertainties, we test the models under variations of these parameters. Our models reveal elevated heat flow distributions ranging between 50 mW/m² and 100 mW/m² and further advance the understanding of the thermal crustal state in this sector.

The thermal state of polar continental crust plays a crucial role for understanding the stability and thickness of large ice sheets, the visco-elastic response of the solid Earth due to unloading when large ice caps melt and, in turn, the accuracy of future sea-level rise prediction. Various studies demonstrate the need for precise measurements and estimation of geothermal heat flow (GHF) in Antarctica for better constrained boundary conditions to enhance the ice sheet model performance. This study provides ground-truth for regional indirect GHF estimates in the Amundsen Sea Embayment, which is part of the West Antarctic Rift System, by presenting in situ temperature measurements in continental shelf sediments. Our results show regionally elevated and heterogeneous GHF (mean of 65 mWm−2) in the Amundsen Sea Embayment. Considering thermal blanketing effects, induced by inflow of warmer water and sedimentary processes, the estimated GHF ranges between 65 mWm−2and 95 mWm−2.

The West Antarctic Rift System is one of the least understood rift systems on earth, but displays a unique coupled relationship between tectonic processes and ice sheet dynamics. Geothermal heat flux (GHF) is a poorly constrained parameter in Antarctica and suspected to affect basal conditions of ice sheets, i.e. basal melting and subglacial hydrology. Thermomechanical models demonstrate the influential boundary condition of geothermal heat flux for (paleo) ice sheet stability. Young, continental rift systems are regions with significantly elevated geothermal heat flux (GHF), because the transient thermal perturbation to the lithosphere caused by rifting requires ∼100 Ma to reach long-term thermal equilibrium. We discuss airborne, high-resolution magnetic anomaly data from the Amundsen Sea Sector, to provide additional insight into deeper crustal structures related to the West Antarctic Rift System in the Amundsen/Bellingshausen sector. With the depth-to-the-bottom of the magnetic source (DBMS) estimates we reveal spatial changes at the bottom of the igneous crust and the thickness of the magnetic layer, which can be further incorporated into tectonic interpretations. The DBMS also marks an important temperature transition zone of approximately 580◦C and therefore serves as a boundary condition for our numerical FEM thermal models in 2D and 3D.

The ice sheet inWest Antarctica is underlain by theWest Antarctic Rift System, which yields critical geological boundary conditions. The bedrock geology and the crustal structure of the rift system may influence the dynamics of the overlying glaciers, which in turn affect the stability of the ice sheet. Previous geophysical surveys have traced the West Antarctic Rift System from the Ross Sea to the Bellingshausen Sea and compared it to other major continental rift zones, such as the East African Rift System or the Basin and Range Province. While the rift system in the Ross Sea sector is relatively well understood, the remaining part of the rift system surrounds a higher degree of uncertainty. Young, continental rift systems, such as the West Antarctic Rift System, are associated with high geothermal heat flow and elevated lithospheric geotherms. In-situ temperature observations of geothermal heat flow are extremely sparse in Antarctica, but present crucial thermal boundary conditions ice sheet models and related sea level rise predictions. Moreover, temperature measurements are urgently required to study geodynamic and tectonic processes, subglacial lakes, hydrologic networks and ecosystems beneath ice sheets, that remain largely unexplored. Indirect methods, that estimate geothermal heat flow on regional to continental scales show poor correlation, which leads to ambiguous results in e.g. ice sheet models. Scientifically, this project aims at contributing to the overall knowledge of the thermal state of the crust in the Amundsen Sea Sector. Within the context of this thesis, a novel suit of in-situ temperature measurements were collected in the Amundsen Sea Embayment during RV Polarstern expedition PS75 (2010) and PS104 (2017). A novel magnetic anomaly grid is further presented, which includes aeromagnetic data collected during RV Polarstern expedition PS104, as well as previous aeromagnetic surveys, and forms the base for investigations of the thermal state of the

Due to a complex tectonic and magmatic history of West Antarctica, the region is suspected to exhibit strong heterogeneous geothermal heat flux variations. Although the maximum ice extent has retreated from the shelf since the last glacial maximum, the trends of offshore GHF patterns and the overall order of magnitude are hypothetically related to those areas onshore where the West Antarctic Ice Sheet (WAIS) rests on geologically related structures. High-resolution GHF will aid the understanding of the paleo-retreat of the ice sheet in the Amundsen Sea Sector. This presentation builds on our previous studies in which we discussed geothermal heat flux based on 26 in-situ temperature measurements that were conducted in 2010 in the Amundsen Sea Embayment (ASE) in West Antarctica. We found, that the shallow (3 m) in-situ temperature measurements were likely influenced by inter-annual bottom-water temperature variability, leading to GHF estimates biased towards lower values (mean = 33 mWm-²). During RV Polarstern expedition PS104 in early 2017 we collected additional 28 in-situ temperature measurements in marine sediments (11 m) for deriving geothermal heat flux in the ASE, which will overall improve the spatial coverage of this region. Furthermore, we monitored the vertical temperature profile of the water column at these stations, which allows to map Circumpolar Deep Water (CDW) distributions across the inner Pine Island Shelf with greater detail.

The West Antarctic Rift System is one of the least understood rift systems on earth, but displays a unique coupled relationship between tectonic processes and ice sheet dynamics. Geothermal heat flux (GHF) is a poorly constrained parameter in Antarctica and suspected to affect basal conditions of ice sheets, i.e., basal melting and subglacial hydrology. Thermomechanical models demonstrate the influential boundary condition of geothermal heat flux for (paleo) ice sheet stability. Young, continental rift systems are regions with significantly elevated geothermal heat flux (GHF), because the transient thermal perturbation to the lithosphere caused by rifting requires ~100 Ma to reach long-term thermal equilibrium. We discuss airborne, high-resolution magnetic anomaly data from the Amundsen Sea Sector, to provide additional insight into deeper crustal structures related to the West Antarctic Rift System in the Amundsen/Bellingshausen sector. With the depth-to-the-bottom of the magnetic source (DBMS) estimates we reveal spatial changes at the bottom of the igneous crust and the thickness of the magnetic layer, which can be further incorporated into tectonic interpretations.

The West Antarctic Rift System is one of the least understood rift systems on earth, but displays a unique coupled relationship between tectonic processes and ice sheet dynamics. Geothermal heat flux is a poorly constrained parameter in Antarctica and suspected to affect basal conditions of ice sheets, i.e., basal melting and subglacial hydrology. Thermomechanical models demonstrate the influential boundary condition of geothermal heat flux for (paleo-)ice sheet stability. Young, continental rift systems are regions with significantly elevated geothermal heat flux, because the transient thermal perturbation to the lithosphere caused by rifting requires ~100 Ma to reach long-term thermal equilibrium. We discuss airborne, high-resolution magnetic anomaly data from the Amundsen Sea sector to provide additional insight into deeper crustal structures related to the West Antarctic Rift System in the Amundsen Sea sector. Using depth-to-the-bottom of the magnetic source (DBMS) estimates, we reveal spatial changes at the bottom of the igneous crust and the thickness of the magnetic layer, which can be further incorporated into tectonic interpretations and which is used to derive geothermal heat flux, supplemented by heat flux derived from measured temperature gradients in shelf sediments. We relate the distribution of geothermal heat flux to paleo and present ice sheet flow conditions.

Elevated and/or spatially variable geothermal heat flux (GHF) is suspected to affect basal conditions of ice sheets, i.e. basal melting and subglacial hydrology. Thermomechanical models demonstrate the influential boundary condition of geothermal heat flux for (paleo) ice sheet stability. Due to a complex tectonic and magmatic history of West Antarctica, the region is suspected to exhibit strong heterogeneous geothermal heat flux variations [e.g. Schroeder et al., 2014; Fisher et al., 2015]. Although the maximum ice extent has retreated from the shelf since the last glacial maximum, the trends of offshore GHF patterns and the overall order of magnitude are hypothetically related to those areas onshore where the West Antarctic Ice Sheet (WAIS) rests on geologically related structures. High-resolution GHF will aid the understanding of the paleo-retreat of the ice sheet in this sector. The problem with testing these possibilities is that direct observations of GHF in Antarctica are so sparse that it is accounted for the greatest source of uncertainty in ice sheet studies for the continent [Larour et al., 2012]. This presentation builds on our previous studies in which we discussed geothermal heat flux based on 26 in-situ temperature measurements that were conducted in 2010 in the Amundsen Sea Embayment (ASE) in West Antarctica. We found, that the shallow (3 m) in-situ temperature measurements were likely influenced by inter-annual bottom-water temperature variability, leading to GHF estimates biased towards lower values (mean = 33 mWm-²). In contrast, our numerical models of geothermal heat fluxes, based on Depth-to-the-Bottom-of-the-Magnetic-Source estimates, suggest that GHF spatially varies from 68 to 110 mWm-². During RV Polarstern expedition PS104 in early 2017 we collected additional 28 in-situ temperature measurements in marine sediments (up to 11 m probe depth) for deriving geothermal heat flux in the ASE, which will overall improve the spatial coverage of this

The West Antarctic Rift System is one of the least understood rift systems on earth, but displays a unique coupled relationship between tectonic processes and ice sheet dynamics. Geothermal heat flux (GHF) is a poorly constrained parameter in Antarctica and suspected to affect basal conditions of ice sheets, i.e., basal melting and subglacial hydrology. Thermomechanical models demonstrate the influential boundary condition of geothermal heat flux for (paleo) ice sheet stability. Young, continental rift systems are regions with significantly elevated geothermal heat flux (GHF), because the transient thermal perturbation to the lithosphere caused by rifting requires ~100 Ma to reach long-term thermal equilibrium. We discuss airborne, high-resolution magnetic anomaly data from the Amundsen Sea Sector, to provide additional insight into deeper crustal structures related to the West Antarctic Rift System in the Amundsen/Bellingshausen sector. With the depth-to-the-bottom of the magnetic source (DBMS) estimates we reveal spatial changes at the bottom of the igneous crust and the thickness of the magnetic layer, which can be further incorporated into tectonic interpretations. The DBMS also marks an important temperature transition zone of approximately 580°C and therefore serves as a boundary condition for our numerical FEM thermal models in 2D and 3D.

Elevated and/or spatially variable geothermal heat flux (GHF) is suspected to affect basal conditions of ice sheets, i.e. basal melting and subglacial hydrology. Thermomechanical models demonstrate the influential boundary condition of geothermal heat flux for (paleo) ice sheet stability. Due to a complex tectonic and magmatic history of West Antarctica, the region is suspected to exhibit strong heterogeneous geothermal heat flux variations [e.g. Schroeder et al., 2014; Fisher et al., 2015]. Although the maximum ice extent has retreated from the shelf since the last glacial maximum, the trends of offshore GHF patterns and the overall order of magnitude are hypothetically related to those areas onshore where the West Antarctic Ice Sheet (WAIS) rests on geologically related structures. High-resolution GHF will aid the understanding of the paleo-retreat of the ice sheet in this sector. The problem with testing these possibilities is that direct observations of GHF in Antarctica are so sparse that it is accounted for the greatest source of uncertainty in ice sheet studies for the continent [Larour et al., 2012]. This presentation builds on our previous studies in which we discussed geothermal heat flux based on 26 in-situ temperature measurements that were conducted in 2010 in the Amundsen Sea Embayment (ASE) in West Antarctica. We found, that the shallow (3 m) in-situ temperature measurements were likely influenced by inter-annual bottom-water temperature variability, leading to GHF estimates biased towards lower values (mean = 33 mWm-²). In contrast, our numerical models of geothermal heat fluxes, based on Depth-to-the-Bottom-of-the-Magnetic-Source estimates, suggest that GHF spatially varies from 68 to 110 mWm-². During RV Polarstern expedition PS104 in early 2017 we collected additional 28 in-situ temperature measurements in marine sediments (up to 11 m probe depth) for deriving geothermal heat flux in the ASE, which will overall improve the spatial coverage of this

The West Antarctic Rift System (WARS) is one of the largest continental rifts globally, but its lateral extent, distribution of local rifts, timing of rifting phases, and mantle processes are still largely enigmatic. It has been presumed that the rift and its crustal extensional processes have widely controlled the history and development of West Antarctic glaciation with an ice sheet of which most is presently based at sub-marine level and which is, therefore, likely to be highly sensitive to ocean warming. While the western domain of the WARS in the Ross Sea has been studied in some detail, only recently have various geophysical and geochemical/thermochronological analyses revealed indications for its eastern extent in the Amundsen Sea and Bellingshausen Sea sectors of the South Pacific realm and in the eastern Marie Byrd Land, Ellsworth Land, Thurston Island and Antarctic Peninsula crustal blocks. One of the current models, based on these studies and additional data, suggests that the WARS activity included tectonic translateral, transtensional and extensional processes from the Amundsen Sea Embayment to the Bellingshausen Sea region of the southern Antarctic Peninsula, basically following the eastward migrating collision of the Phoenix Plate with the Antarctic Plate. We present the range of existing and novel hypotheses regarding the extent of the eastern WARS as well as published and yet unpublished geophysical and geological data, including geothermal heatflow, that support a conceptual WARS model for West Antarctica with implications for glacial onset and developments.

The West Antarctic Rift System (WARS) is one of the largest continental rifts globally, but its lateral extent, distribution of local rifts, timing of rifting phases, and mantle processes are still largely enigmatic. It has been presumed that the rift and its crustal extensional processes have widely controlled the history and development of West Antarctic glaciation with an ice sheet of which most is presently based at sub-marine level and which is, therefore, likely to be highly sensitive to ocean warming. While the western domain of the WARS in the Ross Sea has been studied in some detail, only recently have various geophysical and geochemical/thermochronological analyses revealed indications for its eastern extent in the Amundsen Sea and Bellingshausen Sea sectors of the South Pacific realm. The current model, based on these studies and additional data, suggests that the WARS activity included tectonic translateral, transtensional and extensional processes from the Amundsen Sea Embayment to the Bellingshausen Sea region of the southern Antarctic Peninsula. We present the range of existing hypotheses regarding the extent of the eastern WARS as well as published and yet unpublished data that support a conceptual WARS model for the eastern West Antarctica with implications for glacial onset and developments.

The West Antarctic Rift System (WARS) is one of the largest continental rifts globally, but its lateral extent, distribution of local rifts, timing of rifting phases, and mantle processes are still largely enigmatic. It has been presumed that the rift and its crustal extensional processes have widely controlled the history and development of West Antarctic glaciation with an ice sheet of which most is presently based at sub-marine level and which is, therefore, likely to be highly sensitive to ocean warming. While the western domain of the WARS in the Ross Sea has been studied in some detail, only recently have various geophysical and geochemical/thermochronological analyses revealed indications for its eastern extent in the Amundsen Sea and Bellingshausen Sea sectors of the South Pacific realm and in the eastern Marie Byrd Land, Ellsworth Land, Thurston Island and Antarctic Peninsula crustal blocks. One of the current models, based on these studies and additional data, suggests that the WARS activity included tectonic translateral, transtensional and extensional processes from the Amundsen Sea Embayment to the Bellingshausen Sea region of the southern Antarctic Peninsula, basically following the eastward migrating collision of the Phoenix Plate with the Antarctic Plate. We present the range of existing and novel hypotheses regarding the extent of the eastern WARS as well as published and yet unpublished data that support a conceptual WARS model for West Antarctica with implications for glacial onset and developments.

The West Antarctic Rift System (WARS) provides a key component to Antarctica's tectonic evolution. The system's expansion is thought to progress from the Ross Sea to the Bellingshausen Sea and Amundsen Sea, where smaller rift arms seem to spread more diffusely. The rift troughs provide pathways for major ice streams, thus, their dynamics might directly be coupled to tectonic-morphological constraints. To-gether with regional crustal uplift (e.g. Marie Byrd Land), a common process in conti-nental rift systems, this has widely shaped the West Antarctic landscape during the Neogene. In the Amundsen Sea Embayment (ASE), a key sector of the West Antarctic Ice Sheet (WAIS), rapid changes have occurred over recent decades. The adjacent Pine Island Glacier and Thwaites Glacier, two outlets from a large drainage basin in the centre of the WAIS, exhibit highest increase in flow velocity in all of Antarctica. A large fraction of the WAIS is discharged into the embayment here. However, various models have been developed on the crustal architecture and tectonic history of this region, based on recent geophysical surveys, but any possible WARS activity re-mains uncertain. To investigate the possible effects of rifting history from the WARS on the ASE ice sheet dynamics, we use Curie Point Depth (CPD) estimates. They are based on air-borne-magnetic anomaly data and provide an additional insight into the deeper cru-stal properties. The CPD estimates image the depth of the deepest magnetic layer, hence the bottom depth of the igneous crust. For our estimates we assume a Curie temperature of 580°C at this depth. The well-established centroid method is used to calculate 30 CPDs in area windows of 200 x 200 km each with 50% overlapping the magnetic anomaly grid. We find that shallow CPDs and, therefore, higher geothermal gradients coincide with the location of previously postulated rift arms on the ASE shelf. Our in-situ tempera-ture measurements and derived geothermal heat flow provid